Energy and Work
Types of Energy Kinetic Energy Potential Energy
Forms of Energy Chemical Sound Radiant Electrical Mechanical Magnetic Thermal Nuclear
Radiant Energy Radiant energy -electromagnetic energy. Radiant energy is movement of photons. Examples -
Chemical Energy Chemical energy -energy stored in chemical bonds. This is potential energy until the bonds are broken. Examples - petroleum, natural gas, propane, and coal.
Electrical Energy Electrical energy - movement of elections. Examples - Lightning and static electricity. Science hasn't found a way to use electrical energy, like lightning.
Nuclear Energy Nuclear energy is energy stored in the nucleus of an atom. Nuclear energy gives off light / heat. Breakdown of elements produces energy.
Thermal Energy Thermal energy is heat energy. Caused by vibration of atoms and molecules. Examples - Boiling water, burning wood, and rubbing hands together.
Sound Energy Sound energy - movement of molecules in the air producing vibrations. Examples - Alarms, music, speech, ultrasound medical equipment.
Potential energy + Kinetic energy = Mechanical energy Mechanical energy is movement of machine parts. Mechanical energy is the amount of kinetic and potential energy in a system. Examples - Wind-up toys, grandfather clocks, and pogo sticks. Potential energy + Kinetic energy = Mechanical energy Next
Mechanical Energy Potential energy + Kinetic energy = Mechanical energy Example of energy changes in a swing or pendulum. Forms of Energy
Magnetic Energy Magnetic energy is the attraction of objects made of iron. Examples - Medical equipment, compass, refrigerator magnets
Potential Energy Potential energy exists whenever an object with mass has a position within a force field. Example: Gravity The potential energy of an object in this case is given by the relation: PE = mgh PE = Energy (in Joules) m = mass (in kilograms) g = gravitational acceleration of the earth (9.8 m/sec2) h = height above earth's surface (in meters)
Joules are units for energy What are Joules? Joules are units for energy PE = mgh m= kg g = m/s/s h = m 2 joules = kg m s
Potential Energy The potential energy of the 100-N boulder with respect to the ground below is 200 J in each case. The boulder is lifted with 100 N of force. The boulder is pushed up the 4-m incline with 50 N of force. The boulder is lifted with 100 N of force up each 0.5-m stair.
Important points Gravitational Potential energy is only calculated using the height above the ground, NOT distance traveled to get there. Elastic potential energy is energy in a spring because it bounces back. Also bow in a bow and arrow.
Kinetic Energy Kinetic energy is energy of motion. KE = (1/2)mv2 m=mass of the object V=velocity of the object The greater the mass or velocity of a moving object, the more kinetic energy it has.
Kinetic Energy The greater the mass or velocity of a moving object, the more kinetic energy it has.
Work Work is the transfer of energy through motion. For vertical work to take place, a force must be exerted through a VERTICAL distance. Work=force x distance
Work Clear definition of work – In order for a force to qualify as having done work on an object, there must be a displacement in the same direction as the force. Is this an illustration of work or no work?
Answer No work is being done because the displacement is in a different direction than the force.
Work Work is done on the books when they are being lifted, but no work is done on them when they are being held or carried horizontally because the force is upward. Main Menu
One joule (J) of work is done when a force of 1 N is exerted over a distance of 1 m (lifting an apple over your head).
So Joules are the units for energy and for work! Time to see if you can relate them!
Work Energy Theorem
Work-Energy Theorem The work-energy theorem states that whenever work is done, energy changes.
Work-Energy Theorem To change the kinetic energy of an object, work must be done on the object. If an object is moving, work is required to bring it to rest. SO ……. Work = Kinetic Energy
The amount of kinetic energy dictates how far it skids. Work-Energy Theorem think! When the brakes of a car are locked, the car skids to a stop. How much farther will the car skid if it’s moving 3 times as fast? The amount of kinetic energy dictates how far it skids.
Work-Energy Theorem think! When the brakes of a car are locked, the car skids to a stop. How much farther will the car skid if it’s moving 3 times as fast? Answer: Nine times farther. The car has nine times as much kinetic energy when it travels three times as fast:
The KE in each case = the work needed to stop the car Practice Problems The KE in each case = the work needed to stop the car Assume mass of 500 Kg for car. If a car has a velocity of 10 m/s north, how much kinetic energy does it have? If a car has a velocity of 20 m/s north, how much kinetic energy does it have? If a car has a velocity of 30 m/s north, how much kinetic energy does it have?
Work-Energy Theorem Typical stopping distances for cars equipped with antilock brakes traveling at various speeds. The work done to stop the car is friction force × distance of slide.
Work-Energy Theorem Work equals the change in kinetic energy. Work = ∆KE The work in this equation is the net work—that is, the work based on the net force. Force of Engine The net work is what makes the car go forward in spite of friction pushing backward. The engine must do more work than the friction in order to achieve forward motion.
Work-Energy Theorem If there is no change in an object’s kinetic energy, then no net work was done on it. Push against a box on a floor. If it doesn’t slide, then you are not doing work on the box. On a very slippery floor, if there is no friction at all, the work of your push times the distance of your push appears as kinetic energy of the box.
Law of Conservation of Energy Law of Conservation of Energy- Energy can neither be created nor destroyed. Energy is always changing from one kind to another. The total energy of an object never changes. Potential energy + Kinetic energy = Total energy
Conservation of Energy Part of the PE of the wound spring changes into KE. The remaining PE goes into heating the machinery and the surroundings due to friction. No energy is lost.
Conservation of Energy When the woman leaps from the burning building, the sum of her PE and KE remains constant at each successive position all the way down to the ground.
Power Power is the rate at which work is done. P = work/Dt The unit of power is a watt (joule/sec).
Power The unit of power is the joule per second, also known as the watt. One watt (W) of power is expended when one joule of work is done in one second. One kilowatt (kW) equals 1000 watts. One megawatt (MW) equals one million watts.
Power think! If a forklift is replaced with a new forklift that has twice the power, how much greater a load can it lift in the same amount of time? If it lifts the same load, how much faster can it operate?
Power think! If a forklift is replaced with a new forklift that has twice the power, how much greater a load can it lift in the same amount of time? If it lifts the same load, how much faster can it operate? Answer: The forklift that delivers twice the power will lift twice the load in the same time, or the same load in half the time.
Assessment Questions Raising an auto in a service station requires work. Raising it twice as high requires half as much work. the same work. twice the work. four times the work.
Assessment Questions Raising an auto in a service station requires work. Raising it twice as high requires half as much work. the same work. twice the work. four times the work. Answer: C
Assessment Questions Raising an auto in a service station requires work. Raising it in half the time requires half the power. the same power. twice the power. four times the power.
Assessment Questions Raising an auto in a service station requires work. Raising it in half the time requires half the power. the same power. twice the power. four times the power. Answer: C
conservation of energy. Assessment Questions The energy due to the position of something or the energy due to motion is called potential energy. kinetic energy. mechanical energy. conservation of energy.
conservation of energy. Answer: C Assessment Questions The energy due to the position of something or the energy due to motion is called potential energy. kinetic energy. mechanical energy. conservation of energy. Answer: C
After you place a book on a high shelf, we say the book has increased Assessment Questions After you place a book on a high shelf, we say the book has increased elastic potential energy. chemical energy. kinetic energy. gravitational potential energy.
After you place a book on a high shelf, we say the book has increased Assessment Questions After you place a book on a high shelf, we say the book has increased elastic potential energy. chemical energy. kinetic energy. gravitational potential energy. Answer: D
more than four times the KE Assessment Questions An empty truck traveling at 10 km/h has kinetic energy. How much kinetic energy does it have when it is loaded so its mass is twice, and its speed is increased to twice? the same KE twice the KE four times the KE more than four times the KE
more than four times the KE Answer: D Assessment Questions An empty truck traveling at 10 km/h has kinetic energy. How much kinetic energy does it have when it is loaded so its mass is twice, and its speed is increased to twice? the same KE twice the KE four times the KE more than four times the KE Answer: D
Assessment Questions Which of the following equations is most useful for solving a problem that asks for the distance a fast-moving crate slides across a factory floor in coming to a stop? F = ma Ft = ∆mv KE = 1/2mv2 Fd = ∆1/2mv2
Assessment Questions Which of the following equations is most useful for solving a problem that asks for the distance a fast-moving crate slides across a factory floor in coming to a stop? F = ma Ft = ∆mv KE = 1/2mv2 Fd = ∆1/2mv2 Answer: D
the same as its potential energy at that point. negligible. Assessment Questions A boulder at the top of a vertical cliff has a potential energy of 100 MJ relative to the ground below. It rolls off the cliff. When it is halfway to the ground its kinetic energy is the same as its potential energy at that point. negligible. about 60 MJ. more than 60 MJ.
the same as its potential energy at that point. negligible. Assessment Questions A boulder at the top of a vertical cliff has a potential energy of 100 MJ relative to the ground below. It rolls off the cliff. When it is halfway to the ground its kinetic energy is the same as its potential energy at that point. negligible. about 60 MJ. more than 60 MJ. Answer: A